It is generally known that thermoforming is a process that enables molding a heated and soften sheet of thermoplastic material by applying vacuum suction through a perforated mold. The suction makes the sheet adhere to the mold surface. The sheet is then cooled down with the possible assistance of blown cooling air, keeping the shape of the mould. Twin-sheet thermoforming generally refers to the molding of a first sheet in an upper mold and a second sheet in a lower mold, followed by an operation of compression of the two formed sheets against each other while still hot and relatively soft, thereby providing a fused interface to produce a hollow type product.
U.S. Pat. No. 3,398,434, issued to Alesi jr et al. on Aug. 27, 1968, and entitled “Vacuum Forming Apparatus” teaches a typical structure and process usable to carry out such a twin-sheet molding operation.
Although twin-sheet thermoforming has been successfully used for decades to manufacture hollow products such as gas tanks, pallets, water crafts, etc., it has been found desirable to further provide some products with inserts to improve the structure and/or certain properties of the product. An insert can be defined as a body that is not subjected to molding and which has to be inserted between the molded sheets and generally within a cavity created inside the thermoformed hollow product. For example, a metallic frame, tensile cables or a wooden core may be integrated inside a product to improve rigidity; a solid foam core may be integrated to provide sound proofing or thermal insulation. Hardware elements, fasteners and window glasses, for example, may also be integrated in the molded product to avoid subsequent assembly steps whenever possible. Although blow molding and rotomolding processes are currently used for the production of hollow parts, these processes do not allow inclusion of an insert during the molding process. In addition, injection molding of hollow parts with an insert require very complex and expensive tooling and therefore is not appropriate for large parts and/or low volume production of parts.
Accordingly, some solutions have been provided in the prior art to include insert placement during the thermoforming process. Although a traditional approach is to form upper and lower sheets separately, position the insert therebetween and thereafter assemble top and bottom parts with solvent, adhesive or fuse welding, an obvious gain of productivity can result from combining theses steps into a single operation, carried-out at a single molding station. However, existing solutions of that type are found in a very limited range of applications given their generally poor overall performance and high manufacturing cost, considering namely the high level of automation required.
In U.S. Pat. No. 6,705,853 entitled “Six Station Rotary Thermoforming Machine” granted to Nehring on Mar. 16, 2004, the disclosed machine comprises a top panel loading station, a lower panel loading station, two adjacent heating stations, a thermoforming station, an unloading station and a carousel for transferring thermoformable panels between the stations. The thermoforming station includes a pair of opposed, vertically translatable platens which receive respective molds which engage and form the panels and may include a robotic device for loading preformed cores or inserts. In spite of prohibitive cost and size for many facilities and applications, this machine still suffers from major limitations. Since it uses an external automated device to pick and place the insert once the panels have been formed, panel thickness shall be limited to a fairly low value given the very short cooling time of thick panels. The robot only has a few seconds to approach, position the insert, release it and return before the lower platen shall be raised to compress and fuse the formed panels together along their contact edges. This limits usable panel thicknesses, compromises panel interface welding quality and also limits insert structural and positioning complexity.
Similarly, U.S. Pat. No. 5,758,855 entitled “Pallet with Flexible Tensile Reinforcement and method for Making the Same” issued to Jordan et al. on Jun. 2, 1998 teaches a thermoformed reinforced pallet in which a reinforcing tensile member that can be in the form a mesh affixed between upper and lower decks of the pallet body. According to the disclosed method, first and second thermoplastic sheets are heated, then vacuum formed to form upper and lower decks, the reinforcing member is affixed to one of the formed sheets and pre-loaded, and the formed sheets are finally pressed against one another to provide a thermal bonding and capture the insert therebetween. In this method, sheets are heated and formed in sequence and a robot is used to transport and position the flexible tensile member or mesh on posts formed in the lower deck. Upper deck is then pressed down upon lower deck forming a plurality of knit points. Residual heat and pressure integrally fuse decks about their perimeter, posts and other discrete points, creating knit points and capturing the reinforcing insert. Obviously, the machine and process of Jordan et al. suffers from limitations similar to those of Nehring, discussed above.
In U.S. Pat. No. 5,197,396 entitled “Double Deck Plastic Pallet”, and granted to Breezer et al. on Mar. 30, 1993, there is disclosed a plastic pallet having a twin sheet thermoformed upper deck reinforced with a tubular metal substrate, and a twin sheet thermoformed lower deck assembled to the upper deck. Although this patent is pointing out the benefits of providing a metallic substrate insert into the thermoplastic structure, between upper and lower sheets of the upper deck, it fails to teach an appropriate method to perform such a placement efficiently during the twin sheet thermoforming process.